When choosing an inductor in a switching circuit (e.g., boost/buck converter), there are some concerns including inductance, DC-rating current and direct current resistance (DCR) value. DC-rating current is one of the basic characteristics of an inductor, which represents the maximum current allowed to flow through the inductor. DC-rating current is determined by the inductance, inductor size, and wire, etc. For a given inductor size, the DCR value is directly proportional to the inductance and the DC-rating current is inversely proportional to the inductance.
For a DC/DC boost converter, the efficiency during a light load condition is also directly proportional to the inductance. In order to improve the efficiency, there is a need to choose an inductor with higher inductance. However, as noted above, the higher the inductance, the lower the allowed DC-rating current will be. The peak current through the inductor is equal to the summation of the average current (maximum load current) and the peak ripple current of the inductor. As such, an inductor with higher inductance may not meet requirements when used under heavy load conditions. This is because the peak current through the inductor significantly increases as the average current (maximum load current) increases during the heavy load condition and the current through the inductor may exceed the allowed DC-rating current limit. Therefore, an inductor with higher inductance may not be suitable for a high current/heavy load condition.
There are at least two requirements in choosing an inductor in a boost/buck DC/DC converter. First, the inductor needs to be able to work during the high current condition (the peak current of the inductor should be within the allowed DC-rating current limit). Second, the inductor needs to have high inductance to achieve high efficiency under the low current condition. Since the DC-rating current is inversely proportional to the inductance, it is difficult to choose an inductor that meets both requirements.